7354 Kinetic Studies of the Reactions of Oxide, Hydroxide, Alkoxide, Phenyl, and Benzylic Anions with Methyl Chloride in the Gas Phase at 22.5' Diethard K. Bohme" and L. Brewster Young Contribution from the Centre for Research in Experimental Space Science, York Unicersity, Downsciew, Toronto, Canada, and Mobil Chemical Company, Edison, New Jersey 0881 7. Received June 15, 1970 Abstract: As part of a program directed toward an understanding of the intrinsic nature of anion-molecule re- actions involving organic constituents, rate constants have been measured, reaction channels have been identified, and intrinsic reaction probabilities (the ratios of the experimentally determined rate constants to the theoretical collision rate constants) have been calculated for the reactions of oxide, hydroxide, alkoxide, phenyl, and benzylic anions with methyl chloride. All measurements were made in the gas phase at 22.5" under thermal equilibrium conditions using the flowing afterglow technique. The highly charge-localized anions were found to react rapidly with methyl chloride with rate constants larger than 8 x 10-10 cm3molecule-1 sec-1 whereas the charge-delocalized anions reacted only slowly with rate constants less than 3 x 10-11 cm3molecule-1 sec-l. The formation of chloride ion was the dominant reaction channel observed in all cases. Calculated reaction probabilities were larger than 0.20 for the reactions involving highly charge-localized anions and less than 0.03 for the charge-delocalized anion re- actions. These results obtained in a rarified medium provide evidence for a correlation well known from solution studies, namely, a correlation between the reactivity of an anion and the nature of the distribution of its charge. Solvated alkoxide ions could be established as the dominant negative ions in a low-pressure helium afterglow. This allowed an investigation of the effect of weak solvation on the rate of reaction of alkoxide ions with methyl chloride. Upon the association of one molecule of the conjugate acid to the alkoxide ion the specific rate for reaction decreased by at least a factor of three. n our continuing studies of the intrinsic kinetics and substrate, and the influence of weak solvation on the I energetics of anion-molecule reactions involving intrinsic kinetics of these reactions. This paper is organic constituents, we have turned our attention to concerned with the determination of kinetic and the intrinsic behavior of nucleophilic substitution mechanistic information for nucleophilic substitution reactions of the type reactions involving organic anions in which the nature A- + BX +X- + AB (1) of the anion is altered while the substrate remains unaltered. In addition, direct investigations are made where X is a halogen atom. It has been noted by in several cases of the effect of weak solvation on the organic chemists for many years that the rates of such kinetics of the unsolvated reactions. Such investi- reactions proceeding in solution can be influenced by the gations represent an important step in the "extrap- dielectric constant of the solvent and that a change in olation" of intrinsic gas-phase results to condensed- solvent may bring about alterations in the mechanism phase chemistry. of these reactions. In order to rationalize completely such solvent effects in nucleophilic substitution reac- Experimental Section tions, it is desirable to obtain kinetic and mechanistic The present experiments were carried out in one of the flowing information for these reactions in the complete absence afterglow systems of the Environmental Science Services Admin- of solvent perturbations and, when possible, under istration Research Laboratories in Boulder, Colo. The details conditions of very weak solvation. Such information of the apparatus, its operation, and the related data analysis have can be obtained from investigations carried out in the been described in detail previously both with reference to studies of gas phase at low pressures under thermal equilibrium thermal energy ion-molecule reactions of interest in relation to the earth's ionosphere2 and with reference to the novel application of conditions. An experimental technique which is suited this technique to negative ion organic chemistry in the gas phase. for such studies, namely the flowing afterglow tech- In the present investigations, helium was again used as the buffer nique, has recently been described by the authors with gas. The steady-state pressures were in the range 0.22-0.25 Torr. reference to reactions of oxide radical ion and hy- The 0- ions were produced directly in the excitation region prin- droxide ion with saturated and unsaturated hydro- cipally by the dissociative ionization reaction (eq 2) and, probably carbons and of carbanions with molecular oxygen. e + Oy--f 0- + O+ + e (2) The availability in a flowing afterglow of a large number to a much lesser extent, the dissociative attachment reaction (eq of both unsolvated and weakly solvated organic anions 3). Hydroxide ions were produced indirectly downstream from provides us with a unique opportunity to investigate e + 02--f 0- + 0 (3) the intrinsic nature of nucleophilic substitution, especially as regards the determination of an order of the excitation region but several milliseconds upstream from the reaction region by the rapid hydrogen atom abstraction reaction intrinsic nucleophilic reactivity, the intrinsic role (eq 4) which has a measured rate constant of 2.6 X crn? mole- played by the nature of both the nucleophile and the * Address correspondence to this author at York University. (2) €. E. Ferguson, Adcan. Electron. Electroll Phj.s., 24, 1 (1968); E. (1) D. I<. Bohme and L. B. Young, J. Arner. Chem. Soc., 92, 3301 E. Ferguson, F. C. Fehsenfeld, and A. L. Schmeltekopf, Adcall. At. ( 19 70). Mol. Phys., 5, 1 (1969). Journal of the American Chemical Society 1 92:25 1 December 16, 1970 7355 0- + NH3 +OH- + NHz (4) Results cule-l sec-’ in the gas phase.3 The alkoxide ions CH30-, CH3- Calculation of Intrinsic Reactivity. The intrinsic CHzO-, (CH3)zCHO-, and (CH&CO- were generated by intro- reactivities of the various anions toward methyl chlo- ducing the vapor of the corresponding alcohol into a flowing after- glow rich in either OH-, 0-,or NH2-. The intrinsic basicity of OH- ride were estimated in terms of a reaction efficiency or has recently been shown to be higher than that of the simple alkox- reaction probability, P, defined as the ratio of the ex- ide i0ns.4,~ Thus, in the gas phase, OH- will abstract a proton from perimentally determined rate constant, kexptl,to the a simple alcohol to form the corresponding conjugate base and theoretical collision rate constant, k,, i.e., P = kexptl/kc. water according to the reaction For a Maxwell-Boltzmann speed distribution of OH- + ROH +RO- + HzO ( 5) reactant ions and molecules, the maximum mean Transfer of only the hydroxylic proton has been demonstrated by collision rate constant can be expressed as the sum of several workers with appropriate deuterium-labeled compounds. 486 two terms according to eq where e is the electronic 0- ions will abstract both a hydrogen atom and a proton from simple alcohols according to the reaction (a)”’ k, = kL + kD = 2ae - + 0- + ROH +OH- + RO (6) /P / -+ RO- + OH (7) (2.rrepd~)(2~/7rkV/’(10) Hydrogen atom transfer occurs almost exclusively from either the charge, p is the reduced mass of the reactants, T is hydroxyl position or the terminal carbon atom whereas proton the gas temperature, a is the polarizability, and p~ the transfer again occurs exclusively from the hydroxyl position.6 The NHz- ion was readily generated in the excitation region by permanent dipole moment of the neutral substrate. electron impact on ammonia at partial pressures of ammonia less The first term reflects the contribution of the ion- than 5 X Torr. However, “2- reacts rapidly with water induced dipole interaction and the second term the impurities to yield OH- via the proton transfer reaction contribution of the ion-permanent dipole interaction to NHz- + HzO OH- + NH3 (8) the calculated collision rate constant. The ion-perma- nent dipole term is derived on the assumption that the The NHz- ion will also abstract a proton from the simple alcohols presumably from the hydroxyl position only. dipole aligns itself or “locks-in’’ as the ion approaches. In each of the present experiments, a sufficient amount of the However, since the recent trajectory calculations of vapor of the alcohol was added into the flowing afterglow to ensure Dugan, et al., lo have indicated that the “locking-in’’ of maximum conversion of OH-, 0-, or NH2- into the alkoxide ion the dipole does not necessarily occur, the permanent upstream from the reaction region. A moderate addition of the dipole term in eq 1 must be regarded as a measure of the alcohol vapors to the flowing afterglow yielded copious amounts maximum contribution of the ion-permanent dipole of the solvated anions CH30-.KHaOH (I? = 1.2), CzH,O-. KzHeOH (12 = 1-3), (CH~)~CHO-.~Z(CH~)~CHOH(12 = l,Z), interaction to the calculated collision rate constant. and (CH&CO-.n(CH&COH (H = 1-3) presumably uia three- The permanent dipole term, therefore, reflects the body ion-molecule association reactions of the type range of values of the mean collision rate constant, the actual value depending on the degree of alignment of RO-.riROH + ROH + He --f RO-.(/f + I)ROH + He (9) the dipole with the electric field of the incoming ion. which are expected to have rate constants of at least 10-28 cm6 As a consequence of these considerations, calculations molecule-* sec-1.7 The phenyl and benzylic carbanions are such strong bases in were made of both the reaction probability P, = common hydroxylic solvents, such as water and alcohol, that they keXptl/k,and the reaction probability PL = kexptl/kl, usually do not exist in appreciable concentrations in the condensed where kL is the value of k, when the permanent dipole phase.